1. Field of the Invention
The present invention relates to an apparatus for purifying exhaust gas from internal combustion engine and, in particular, to an apparatus for purifying the PM (particulate matter) emitted from a diesel engine.
2. Description of Related Art
A diesel engine is usually used for motor vehicles, particularly for large-size motor vehicles. Recently, it is required to reduce nitrogen oxide, carbon monoxide and hydrocarbon as well as the PM in the exhaust gas from the diesel engine. For this purpose, it is considered to improve an engine or optimize a combustion condition for controlling the generation of the PM, and to purify the generated PM in an exhaust gas.
For removing the PM in an exhaust gas, a ceramic honeycomb filter having a number of cell passages (cell channels), a metal filter and a ceramic fiber filter that trap the PM are generally used. However, in the case of using such a filter, the trapped PM gradually blocks the filter and, further, the filter increases the gas-flow resistance and becomes a larger load on the engine. Further, a nano-size PM tends to pass through the filter and is not trapped. In the case that the filter traps the PM in the exhaust gas, it is difficult to sufficiently burn out the trapped PM on the filter using only the thermal energy of the exhaust gas.
It is well known to use an apparatus that provides an electric discharge for purifying an exhaust gas from diesel engine. For example, Japanese Patent No. 2698804 discloses an apparatus comprising a needle electrode, a deflection electrode, and a trapping electrode surrounding them. The apparatus electrically charges the PM in an exhaust gas from a diesel engine by an electric discharge between the electrodes, and thereby traps the PM on the trapping electrode. However, this apparatus is intended to trap the PM and does not burn the trapped PM. Therefore, the apparatus does not sufficiently burn the trapped PM, and an special treatment is necessary to do it. This is because an electric current passes through the metal trapping electrode rather than the PM deposited on it and, therefore, it is impossible to burn the PM by the electric current.
Further, Japanese National Publication No. 2001-511493 describes an exhaust gas purifying apparatus that comprises insulative pellets between electrodes. However, this apparatus is intended to put an electric power supply close to a reaction furnace, in particular to put the electric power supply into an electrically conductive vessel which is grounded (earthed). Therefore, this reference does not disclose the importance of the angle of the electric field to the direction of the exhaust gas flow, and an importance of the insulative body through which the exhaust gas passes.
Japanese Unexamined Patent Publication No. 60-235620 describes a diesel particulate filter carrying a mixture of a platinum group element and an alkali earth metal oxide for burning the PM trapped on the filter. However, this reference does not describe providing an electric field in the filter, and to combine the electric field and the mixture of platinum group element and alkali earth metal oxide.
Therefore, the exhaust gas purifying apparatuses in the prior arts do not sufficiently use an electric field for trapping the PM, or do not sufficiently burn the trapped PM.